U.S. patent application number 13/959995 was filed with the patent office on 2013-11-28 for method of improving electromechanical integrity of cathode coating to cathode termination interfaces in solid electrolytic capacitors.
This patent application is currently assigned to Kemet Electronics Corporation. The applicant listed for this patent is Kemet Electronics Corporation. Invention is credited to John Bultitude, Antony P. Chacko, Randolph S. Hahn, Philip M. Lessner, John E. McConnell, Robert Ramsbottom.
Application Number | 20130314845 13/959995 |
Document ID | / |
Family ID | 49621429 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130314845 |
Kind Code |
A1 |
Chacko; Antony P. ; et
al. |
November 28, 2013 |
Method of Improving Electromechanical Integrity of Cathode Coating
to Cathode Termination Interfaces in Solid Electrolytic
Capacitors
Abstract
A solid electrolytic capacitor is described which comprises an
anode, a dielectric on the anode and a cathode on the dielectric. A
conductive coating is on the cathode wherein the conductive layer
comprises an exterior surface of a first high melting point metal.
An adjacent layer is provided comprising a second high melting
point metal, wherein the first high melting point metal and the
second high melting point metal are metallurgically bonded with a
low melting point metal.
Inventors: |
Chacko; Antony P.;
(Simpsonville, SC) ; McConnell; John E.;
(Simpsonville, SC) ; Ramsbottom; Robert;
(Simpsonville, SC) ; Lessner; Philip M.;
(Simpsonville, SC) ; Hahn; Randolph S.;
(Simpsonville, SC) ; Bultitude; John;
(Simpsonville, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kemet Electronics Corporation |
Simpsonville |
SC |
US |
|
|
Assignee: |
Kemet Electronics
Corporation
Simpsonville
SC
|
Family ID: |
49621429 |
Appl. No.: |
13/959995 |
Filed: |
August 6, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13114433 |
May 24, 2011 |
|
|
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13959995 |
|
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61348318 |
May 26, 2010 |
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Current U.S.
Class: |
361/502 ;
29/25.03; 361/533 |
Current CPC
Class: |
H01G 9/0425 20130101;
H01G 9/012 20130101; H01G 9/048 20130101; H01G 11/26 20130101; H01G
9/04 20130101; H01G 9/0029 20130101 |
Class at
Publication: |
361/502 ;
361/533; 29/25.03 |
International
Class: |
H01G 9/048 20060101
H01G009/048; H01G 9/00 20060101 H01G009/00; H01G 11/26 20060101
H01G011/26 |
Claims
1. A solid electrolytic capacitor comprising: an anode; a
dielectric on said anode; a cathode on said dielectric; a
conductive coating on said cathode wherein said conductive layer
comprises an exterior surface of a first high melting point metal;
and an adjacent layer comprising a second high melting point metal
wherein said first high melting point metal and said second high
melting point metal are metallurgically bonded with a low melting
point metal.
2. The solid electrolytic capacitor of claim 1 wherein said first
high melting point metal and said second melting point metal are
the same.
3. The solid electrolytic capacitor of claim 1 wherein at least one
of said first high melting point metal or said second melting point
metal is selected from the group consisting of copper, silver,
aluminum, gold, platinum, palladium, beryllium, rhodium, nickel,
cobalt, iron and molybdenum or an alloy thereof.
4. The solid electrolytic capacitor of claim 3 wherein at least one
of said first high melting point metal or said second melting point
metal is selected from the group consisting of nickel, copper,
gold, silver, tin, palladium and lead.
5. The solid electrolytic capacitor of claim 4 wherein said cathode
is plated with metal.
6. The solid electrolytic capacitor of claim 1 wherein said low
melting point metal is selected from the group consisting of tin,
antimony, bismuth, cadmium, zinc, gallium, indium, tellurium,
mercury, thallium, selenium and polonium or an alloy thereof.
7. The solid electrolytic capacitor of claim 6 wherein said low
melting point metal is selected from the group consisting of tin,
antimony and indium.
8. The solid electrolytic capacitor of claim 1 where said cathode
comprises a conductive polymer.
9. The solid electrolytic capacitor of claim 8 wherein said
conductive polymer is selected from the group consisting of
polyaniline, polythiophene and polypyrrole.
10. The solid electrolytic capacitor of claim 9 wherein said
conductive polymer is polyethylene dioxythiophene.
11. The solid electrolytic capacitor of claim 1 wherein said
conductive coating further comprises a conductive particle.
12. The solid electrolytic capacitor of claim 11 wherein said
conductive particle is selected from the group consisting of carbon
black, graphite, graphene, carbon nanotubes, metal particles,
carbon coated metal particles and metal coated carbon
particles.
13. The solid electrolytic capacitor of claim 12 wherein said metal
particles are selected from the group consisting of Ag, Cu, Ni, Sn,
In, Bi, Sb, Au and Pd.
14. The solid electrolytic capacitor of claim 12 wherein said
conductive particle comprise a metal coating selected from a high
melting point metal and a low melting point metal.
15. The solid electrolytic capacitor of claim 1 wherein said
adjacent layer is selected from a cathode lead, a mounting tab and
an adjacent electrode.
16. The solid electrolytic capacitor of claim 15 wherein said
cathode lead is a non-ferrous material or a ferrous material.
17. The solid electrolytic capacitor of claim 16 wherein said
non-ferrous material is selected from copper, phosphor bronze,
brass and beryllium copper.
18. A method for forming a capacitor comprising the steps of:
providing an anode; forming a dielectric on said anode; applying a
cathode on said dielectric; plating a high melting point metal on
said cathode; and forming a metallurgical bond between said high
melting point metal and an adjacent layer with a low melting point
metal.
19. The method for forming a capacitor of claim 18 wherein said
adjacent layer comprises a second high melting point metal.
20. The method for forming a capacitor of claim 19 wherein said
first high melting point metal and said second high melting point
metal are the same.
21. The method for forming a capacitor of claim 18 wherein said
high melting point metal is selected from the group consisting of
copper, silver, aluminum, gold, platinum, palladium, beryllium,
rhodium, nickel, cobalt, iron and molybdenum or an alloy
thereof.
22. The method for forming a capacitor of claim 21 wherein said
high melting point metal is selected from the group consisting of
nickel, copper, gold, silver, tin, palladium and lead.
23. The method for forming a capacitor of claim 22 wherein said
cathode is plated with metal.
24. The method for forming a capacitor of claim 18 wherein said low
melting point metal is selected from the group consisting of tin,
antimony, bismuth, cadmium, zinc, gallium, indium, tellurium,
mercury, thallium, selenium and polonium or an alloy thereof.
25. The method for forming a capacitor of claim 24 wherein said low
melting point metal is selected from the group consisting of tin,
antimony and indium.
26. The method for forming a capacitor of claim 18 further
comprising forming a coupon comprising said low melting point
metal.
27. The method for forming a capacitor of claim 26 wherein said
forming of said metallurgical bond between said high melting point
metal and said adjacent layer comprises placing said coupon between
said high melting point metal and an adjacent layer.
28. The method for forming a capacitor of claim 18 wherein said
cathode comprises a conductive polymer.
29. The method for forming a capacitor of claim 28 wherein said
conductive polymer is selected from the group consisting of
polyaniline, polythiophene and polypyrrole.
30. The method for forming a capacitor of claim 29 wherein said
conductive polymer is polyethylene dioxythiophene.
31. The method for forming a capacitor of claim 18 wherein said
adjacent layer is selected from a cathode lead a mounting tab and
an adjacent cathode.
32. The method for forming a capacitor of claim 31 wherein said
cathode lead is plated with said low melting point metal.
33. The method for forming a capacitor of claim 31 wherein said
cathode lead is a metal or alloys with melting point above
300.degree. C.
34. The method for forming a capacitor of claim 31 wherein said
cathode lead is a non-ferrous material or a ferrous material.
35. The method for forming a capacitor of claim 34 wherein said
non-ferrous material is selected from the group consisting of
copper, phosphor bronze, brass and beryllium copper.
36. A capacitor stack comprising: at least two solid electrolytic
capacitors with each solid electrolytic capacitor of said
electrolytic capacitors comprising: an anode; a dielectric on said
anode; a cathode on said dielectric; and a conductive coating on
said cathode wherein said conductive layer comprises an exterior
surface of a high melting point metal; a metallurgical bond between
adjacent exterior surfaces wherein said metallurgical bond
comprises said high melting point metal and a low melting point
metal. an adjacent layer comprising a second high melting point
metal wherein said first high melting point metal and said second
high melting point metal are metallurgically bonded with a low
melting point metal.
37. The capacitor of claim 36 wherein said high melting point metal
is selected from the group consisting of copper, silver, aluminum,
gold, platinum, palladium, beryllium, rhodium, nickel, cobalt, iron
and molybdenum or an alloy thereof.
38. The capacitor of claim 37 wherein said high melting point metal
is selected from the group consisting of nickel, copper, gold,
silver, tin, palladium and lead.
39. The capacitor of claim 38 wherein said cathode is plated with
metal.
40. The capacitor of claim 36 wherein said low melting point metal
is selected from the group consisting of tin, antimony, bismuth,
cadmium, zinc, gallium, indium, tellurium, mercury, thallium,
selenium and polonium or an alloy thereof.
41. The capacitor of claim 40 wherein said low melting point metal
is selected from the group consisting of tin, antimony and
indium.
42. The capacitor of claim 36 where said cathode comprises a
conductive polymer.
43. The capacitor of claim 42 wherein said conductive polymer is
selected from the group consisting of polyaniline, polythiophene
and polypyrrole.
44. The capacitor of claim 43 wherein said conductive polymer is
polyethylene dioxythiophene.
45. The capacitor of claim 36 wherein said adjacent layer is
selected from a cathode lead a mounting tab and an adjacent
cathode.
46. The capacitor of claim 45 wherein said cathode lead is a
non-ferrous material or a ferrous material.
47. The capacitor of claim 46 wherein said non-ferrous material is
selected from the group consisting of copper, phosphor bronze,
brass and beryllium copper.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Continuation-In-Part of pending
U.S. patent application Ser. No. 13/114,433 filed May 24, 2011
which, in turn, claims priority to expired U.S. Provisional
Application No. 61/348,318 filed May 26, 2010.
BACKGROUND
[0002] The present invention is related to an improved method of
forming a solid electrolytic capacitor and an improved capacitor
formed thereby. More specifically, the present invention is related
to a method of improving the electrical and mechanical integrity of
a cathode to a cathode lead or adjacent layer using a metallurgical
adhesive or transient liquid phase sintering (TLPS) conductive
adhesive to form metallurgical bonds thereby allowing for a stacked
array of capacitors and leadless capacitors or leadless stacked
capacitors.
[0003] The construction and manufacture of solid electrolytic
capacitors is well documented. In the construction of a solid
electrolytic capacitor a valve metal typically serves as the anode.
The anode body can be either a porous pellet, formed by pressing
and sintering a high purity powder, or a foil which is etched to
provide an increased anode surface area. An oxide of the anode,
which serves as the dielectric of the capacitor, is typically
electrolytically formed to cover at least a majority of the
surfaces of the anode. The solid cathode electrolyte is typically
chosen from a very limited class of materials, to include manganese
dioxide and intrinsically conductive polymers such as polyaniline,
polypyrrole, polythiophene, etc. The solid cathode electrolyte is
applied so that it covers at least a majority of the dielectric
surfaces. An important feature of the solid cathode electrolyte is
that it can be made more resistive by exposure to high
temperatures. This feature allows the capacitor to heal leakage
sites by Joule heating. The solid electrolyte is typically not
readily adhered to a lead frame or circuit trace, so in addition to
the solid electrolyte the cathode of a solid electrolyte capacitor
typically comprises several layers which are external to the solid
electrolyte to facilitate adhesion. These layers typically include
a carbon layer; a layer containing a highly conductive metal,
typically silver, bound in a polymer or resin matrix; a conductive
adhesive layer such as solder or a silver adhesive which is then
adhered to a highly conductive metal lead frame. It is important
that the solid electrolyte be of sufficient buildup and density to
prevent the layers overlaying the solid electrolyte from
penetrating the solid electrolyte and contacting the dielectric.
One reason for this is that these outer layers do not necessarily
exhibit the healing properties required for a material directly in
contact with the dielectric. Thus, the ability to control the
buildup, morphology, uniformity, and density of the solid
electrolyte is critical to manufacturing a reliable solid
electrolytic capacitor. The various layers of the external cathode
also serve to protect the dielectric from thermo-mechanical damage
that may occur during subsequent processing, board mounting, or
customer use.
[0004] In the case of conductive polymer cathodes the conductive
polymer is typically applied by chemical oxidation polymerization,
electrochemical oxidation polymerization, spray techniques or
dipping in a slurry of preformed polymer with other less desirable
techniques being reported.
[0005] The carbon layer serves as a chemical buffer between the
solid electrolyte and the silver layer. Critical properties of the
carbon layer include adhesion to the underlying layer, wetting of
the underlying layer, penetration of the underlying layer, bulk
conductivity, interfacial resistance, compatibility with the silver
layer, suitable buildup, and suitable mechanical properties.
[0006] The silver layer, or a suitable very high conductive layer,
serves to conduct current to the lead frame from the areas of the
cathode not directly connected to the lead frame. The critical
characteristics of this layer are high conductivity, adhesive
strength to the carbon layer, wetting of the carbon layer, and
suitable mechanical properties. Compatibility with the subsequent
layers employed in the assembly and encapsulation of the capacitor
are also critical.
[0007] An electrically conductive adhesive is used to attach the
cathode layer to a lead frame. The electrical properties of the
capacitor can be affected if the mechanical integrity of the
adhesive/lead frame connection is degraded during assembly and post
assembly processing. The adhesive properties of the conductive
adhesive, the solder coating on the lead frame, the surface
characteristics of the lead frame, the coefficient of thermal
expansion of the lead frame, etc., need to be carefully controlled
in order to obtain durable negative connection integrity. The
adhesive/lead frame interface is subjected to various thermo
mechanical stresses during molding, curing, aging, surface mount
testing, solder reflow, etc. These thermo mechanical stresses, and
the low adhesive strength of the conductive adhesive, often cause a
break in the electrical contact between the cathode and lead frame.
Adhesives with higher adhesive strengths and lower concentration of
conductive particles are able to withstand the stress and maintain
mechanical integrity. However, there is a trade-off between
increasing adhesion and increasing electrical conductivity.
[0008] Conductive adhesives are heavily filled with silver
particles to get maximum conductivity. Increasing the silver
loading will improve the electrical properties but decreases
binder/resin concentration in the adhesive which is detrimental to
adhesion. Increasing the resin portion will increase adhesion but
to the detriment of electrical properties, particularly,
conductivity.
[0009] U.S. Pat. No. 6,972,943 attempts to circumvent the conflict
between adhesion and conductivity of the adhesive by modifying the
lead frame surface. The method of the invention in the patent
provides grooves and holes in the lead frame so as to have good
mechanical integrity between the two surfaces.
[0010] U.S. Pat. No. 6,916,433 attempts to improve performance by
using conductive fillers with dendrites or protrusions to enhance
contact with electrodes and an elastic adhesive resin for enhanced
flexibility. The preferred elastic adhesive is a thermosetting
resin comprising denatured silicon resin with a dispersed epoxy
resin, available from Cemedyne Co. Ltd.
[0011] U.S. Pat. No. 7,495,890 disclosed a method of improving
cathode connection integrity by using secondary adhesives. Although
this method improves the cathode connection integrity, higher
temperature adhesion performance is limited by the thermal
softening temperatures of the polymeric materials in these
adhesives.
[0012] The polymeric resin in these adhesives helps to form
adhesive bonds between the highly conductive cathode layer and the
lead frame. One of the weaknesses of the polymeric resin is that
they tend to degrade at high temperatures which affects the cathode
connection integrity. Another weakness of these metal particle
filled adhesives is that the conduction mechanism is percolation
assisted by forming a connection between binder coated particles.
Due to this binder interference, stable interconnection with the
lead frame or between particles is an issue especially when these
parts are subjected to thermal, mechanical or environmental stress.
On humidity exposure, moisture absorbed by the binders can swell
the binders causing an increase in equivalent series resistance
(ESR) due to increased silver particle to silver particle distance.
Silver migration is another issue when the conductive adhesive is a
silver filled adhesive. Silver migration can lead to an increase in
leakage current and an increase in ESR. Solders can be used for
forming a metallurgical bond between the lead frame and the cathode
layer. However, most of the solders available are not suitable for
high temperature applications either due to their low melting point
or due to the presence of lead (Pb). A need therefore exists for
improved reliability cathode connections for high temperature
applications.
[0013] Through diligent research the present inventors have
developed a method of improving high temperature adhesive strength
between the cathode layer and an adjacent layer.
SUMMARY
[0014] It is an object of the present invention to provide a
capacitor with increase adhesion between the cathode layer and an
adjacent layer which may be a lead frame, an adjacent capacitor, a
mounting tab or a circuit trace.
[0015] It is another object of the present invention to provide a
capacitor with improved high temperature adhesion performance.
[0016] A particular feature of the present invention is the ability
to provide improvements with minor changes to the manufacturing
method and with improved product yields due to improved thermo
mechanical and electrical properties.
[0017] It is another object of the present invention to provide a
capacitor which maintains an electrically stable interface between
the cathode and adjacent layer when exposed to high humidity.
[0018] It is another object of the present invention to provide a
capacitor wherein adhesion between the cathode and adjacent layer
can be done rapidly thereby increasing manufacturing
efficiencies.
[0019] Yet another advantage is provided in the ability to form
stacked capacitors which increases capacitance per unit volume.
[0020] Yet another advantage is provided in the ability to mount
the capacitors directly to a circuit board, without a lead frame,
either individually or as a stack thereby increasing capacitance
per unit volume and minimizing ESR.
[0021] These and other advantages, as will be realized, are
provided in a solid electrolytic capacitor. The capacitor comprises
an anode and a dielectric on the anode. A cathode is on the
dielectric and a conductive coating on said dielectric. A cathode
lead is electrically connected to the conductive coating by an
adhesive selected from the group consisting of a transient liquid
phase sinterable material and polymer solder.
[0022] Yet another embodiment is provided in a method for forming a
capacitor. The method includes the steps of:
providing an anode; forming a dielectric on the anode; applying a
cathode on the dielectric; and electrically connecting a cathode
lead to the cathode with an adhesive selected from the group
consisting of a transient liquid phase sinterable material and
polymer solder.
[0023] Yet another embodiment is provided in a solid electrolytic
capacitor. The capacitor has an anode and a dielectric on the
anode. A cathode is on the dielectric and a conductive coating is
on the cathode. A cathode lead is electrically connected to the
conductive coating by a metallurgical bond formed from a transient
liquid phase sintered material which is preferable formed under
compression.
[0024] Yet another advantage is provided in a solid electrolytic
capacitor comprising an anode, a dielectric on the anode and a
cathode on the dielectric. A conductive coating is on the cathode
wherein the conductive layer comprises an exterior surface of a
first high melting point metal. An adjacent layer is provided
comprising a second high melting point metal, wherein the first
high melting point metal and the second high melting point metal
are metallurgically bonded with a low melting point metal.
[0025] Yet another embodiment is provided in a method for forming a
capacitor comprising the steps of:
providing an anode; forming a dielectric on the anode; applying a
cathode on the dielectric; plating a high melting point metal on
the cathode; and forming a metallurgical bond between the high
melting point metal and an adjacent layer with a low melting point
metal.
[0026] Yet another embodiment is provided in a capacitor stack
comprising at least two solid electrolytic capacitors. Each solid
electrolytic capacitor comprises an anode a dielectric on the anode
and a cathode on the dielectric. A conductive coating is on the
cathode wherein the conductive layer comprises an exterior surface
of a high melting point metal. A metallurgical bond is between
adjacent exterior surfaces wherein the metallurgical bond comprises
the high melting point metal and a low melting point metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a cross-sectional schematic view of an embodiment
of the invention.
[0028] FIG. 2 is a flow chart representation of an embodiment of
the invention.
[0029] FIG. 3 is a schematic representation of an embodiment of the
invention.
[0030] FIGS. 4A and 4B are cross-sectional schematic view of an
embodiment of the invention.
[0031] FIG. 5 is a schematic cross-sectional view of an embodiment
of the invention.
DETAILED DESCRIPTION
[0032] The present invention mitigates the deficiencies of the
prior art by providing a capacitor with an improvement in adhesion
to the cathodic lead frame through the use of a metallurgical
adhesive selected from transient liquid phase sintering and polymer
solder adhesives. The metallurgical adhesives increase productivity
without detriment to the electrical properties of the
capacitor.
[0033] It has now been found that metallurgical adhesives can be
used for attaching solid electrolytic cathode layers to an adjacent
layer such as a lead frame, a mounting pad, a circuit trace or an
adjacent cathode. It has also been found that the metallurgical
adhesives form a metallurgical bond between a cathode layer and
adjacent layer. In a particularly preferred embodiment a solid
electrolytic capacitor with a metal plated cathode, preferably a
nickel plated cathode, forms metallurgical bonds at the interfaces
when the metallurgical adhesives or TLPS are incorporated. The
metal plated layer is preferably applied by reverse bias
electroplating.
[0034] Metallurgical adhesives are conductive adhesives which make
interconnection through metallic bonds instead of chemical bonds as
in metal filled polymeric adhesives. Because the metal particles in
these adhesives are sintered together, these adhesives enable
conduction by metallic conduction instead of percolation assisted
conduction as in metal filled polymeric adhesives. For the purposes
of this disclosure metallurgical adhesives include transient liquid
phase sinterable materials and polymer solder. By using these
metallurgical adhesives, metallurgical bonds can be formed between
the lead frame and the cathode layers.
[0035] Transient liquid phase sinterable adhesives are blends of
low temperature melting metals or alloys and high temperature
melting metals or alloys which can be sinterable at low
temperatures. Transient liquid phase sintering conductive adhesive
formulations disclosed in U.S. Pat. No. 5,853,622 combine TLPS
materials with cross linking polymers to create a thermal and
electrical bond having intermetallic interfaces between the metal
surfaces created by the TLPS process. The spraying of two mating
surfaces with a low temperature melting material on one surface and
a higher melting temperature on the mating surface, with both
surfaces being compatible with the TLPS process, thereby forming a
joint when heating to the melting point of the lower temperature
material is discussed in U.S. Pat. No. 5,964,395. These patents
describe the materials and processes of TLPS with respect to
forming a conductive bond.
[0036] Transient Liquid Phase Sintering adhesives are conductive
materials that are distinguished from solders. Solders are alloys
which do not undergo a change in composition during reflow. TLPS
materials are mixtures of two or more metals or metal alloys prior
to exposure to elevated temperatures. The second distinguishing
characteristic of TLPS materials is that the melting point of the
material is dependent on the thermal history of the material. TLPS
materials exhibit a low melting point prior to exposure to elevated
temperatures, and a higher melting point following exposure to
these temperatures. The initial melting point is the result of the
low temperature metal or an alloy of two low temperature metals.
The second melting temperature is that of the intermetallic formed
when the low temperature metal or alloy, forms a new alloy with a
high temperature melting point metal thereby creating an
intermetallic having a higher melting point. TLPS materials form a
metallurgical bond between the metal surfaces to be joined. Unlike
tin/lead or Pb free solders, the TLPS conductive adhesives do not
spread as they form the intermetallic joint. Rework of the TLPS
system is very difficult due to the high secondary reflow
temperatures.
[0037] A transient liquid phase sinterable adhesive can be used to
attach a plated metal layer of a cathode to a cathode lead or to
attach adjacent plated metal layers such as a mounting tab or
adjacent cathode in a stack of capacitors. Commercially available
transient liquid phase sinterable adhesive used in the electronics
industry are filled with a mixture of low melting metals or alloys
and high temperature melting metals or alloys. In addition to these
sinterable metallic components, some amount of curable organic
materials may be present to provide fluxing action and some initial
tackiness, however, with TLPS a flux is not desirable. Transient
liquid phase sinterable adhesives are available from Ormet Circuits
Inc. and Creative Electron as noted suppliers.
[0038] TLPS comprise high temperature materials selected from
copper, silver, aluminum, gold, platinum, palladium, beryllium,
rhodium, nickel, cobalt, iron and molybdenum or a mixture or any
combination thereof are suitable for use in transient liquid phase
sintering conductive adhesives. High melting temperature materials
have a melting point of at least 600.degree. C.
[0039] TLPS further comprise a low melting temperature materials
selected from tin, antimony, bismuth, cadmium, zinc, gallium,
indium, tellurium, mercury, thallium, selenium, or polonium, or a
mixture or an alloy of any two or more of these. Low melting
temperature materials have a melting point of no more than
500.degree. C.
[0040] A particularly suitable embodiment is either silver or
copper as the high temperature component and a tin-bismuth alloy as
the low temperature component. Another particularly suitable
embodiment is nickel as the high temperature component and tin as
the low temperature component.
[0041] The transient liquid phase sintering conductive adhesives
are compatible with surface finishes containing silver, tin, gold,
copper, platinum, palladium, nickel, or combinations thereof,
either as lead frame finishes, component connections or inner
electrodes to form an electronically conductive metallurgical bond
between two surfaces. Suitable external lead or lead frame
materials include phosphor bronze, copper, alloys of copper such as
but not limited to beryllium copper, Cu194 and Cu192, as well as
lead frames consisting of ferrous alloys such as but not limited to
Alloy 42 and Kovar.
[0042] With transient liquid phase sintering adhesives in paste
form thermocompression bonding can be used to increase densities in
the bond thereby forming more reliable joints than relying on
temperature alone.
[0043] A particular advantage is the ability to use a low process
time of 15 to 60 seconds at a temperature in the range of
225.degree. C. to 300.degree. C. in a single step making it
suitable for automation. Robust joints can be created for the
application of attaching external leads to the cathode or for
attaching adjacent cathodes using Transient Liquid Phase Sintering
conductive adhesives with a one-step low temperature in less than
60 seconds and in combination with thermo-compression bonding.
[0044] A polymer solder can be used to form metallurgical bonds
between the plated metal cathode layer and the cathode lead.
Polymer solder provides suitable solder wetting, particularly, on
plated cathode layers. Thermosetting polymer in combination with
high temperature alloy provides higher temperature properties.
Henkel supplies such adhesives as epoxy solder.
[0045] Polymer solders may consist of conventional solder systems
based on Pb/Sn alloy systems or more preferably lead free systems,
such as Sn/Sb, which are combined with cross linking polymers which
serve as cleaning agents. The cross-linked polymers also have the
ability to form a cross linked polymer bond, such as an epoxy bond,
that forms during the melting phase of the metals thereby forming a
solder alloy and a mechanical polymeric bond. An advantage of
polymer solders is that the polymeric bond provides additional
mechanical bond strength at temperatures above the melting point of
the solder, thus giving the solder joint a higher operating
temperature in the range of about 5 to 30.degree. C. above the
melting point of the solder. Polymer solders combine current solder
alloys with a cross linking polymer within the same paste to
provide both a metallurgical bond and a mechanical bond when cured,
such as by heating, to provide additional solder joint strength at
elevated temperatures. However, the upper temperature limits and
joint strength has been increased, just by the physical properties
of the materials. A practical limit of 300.degree. C. remains
whereas the Transient Liquid Phase Sintering Conductive Adhesives
can achieve higher temperatures.
[0046] Thermo compressive bonding is also a particularly preferred
processing method when using polymer solder because it assists in
the formation of a high density metallurgical bond between the
contacting surfaces. The advantages of a thermo-compression include
a more robust bond with respect to secondary attachment processes
and attachments with higher strength are achieved.
[0047] A compression force of 0.5 to 4.5 Kilograms/cm.sup.2 (7.1 to
64 psi) and more preferably 0.6 to 0.8 Kilograms/cm.sup.2 (8.5 to
11 psi) is sufficient for demonstration of the thermo-compression
teachings herein. About 0.63 Kilograms/cm.sup.2 (9 psi) is a
particularly suitable pressure for demonstration of the
teachings.
[0048] The present invention will be described with reference to
the various figures which illustrate, without limiting, the
invention.
[0049] In FIG. 1, a cross-sectional schematic view of a capacitor
is shown as represented at 10. The capacitor comprises an anode,
11, preferably comprising a valve metal as described herein. A
dielectric layer, 12, is provided on the surface of the anode, 11.
The dielectric layer is preferably formed as an oxide of the valve
metal as further described herein. Coated on the surface of the
dielectric layer, 12, is a conductive layer, 13. The conductive
layer preferably comprises conductive polymer, such as
polyethylenedioxythiophene (PEDT), polyaniline or polypyrrole or
their derivatives; manganese dioxide, lead oxide or combinations
thereof. A carbon layer, 14, a plated layer, 16, provides
electrical conductivity and provide a surface which is more readily
adhered to the cathode terminal, 17, than is the cathode layer, 13.
The metallurgical adhesive layer, 21, secures the cathode lead to
the silver layer or plated layer. The plated layer can be from
sputtered metal, chemical vapor deposited metal or electroplated
metal with reverse bias electroplating most preferable.
[0050] The carbon layer together with the silver layer and adhesive
layer provides a strongly adhered conductive path between the
conductive layer, 13, and the cathode terminal, 17. An anode wire,
18, provides electrical contact between the anode, 11, and an anode
terminal, 19. The entire element, except for the terminus of the
terminals, is then preferably encased in a non-conducting material,
20, such as an epoxy resin.
[0051] The capacitor is illustrated in FIG. 1 as a discrete
capacitor. In an alternate embodiment the anode wire, 18, and
plated layer, 16, may be in direct electrical contact with a
circuit trace wherein elements of the circuit may constitute the
cathode lead, anode lead or both. The capacitor may be embedded in
a substrate or incorporated into an electrical component with
additional functionality.
[0052] The carbon layer comprises a conductive composition
comprising resin and carbon conductive particles. The carbon layer
may also comprise adjuvants such as crosslinking additives,
surfactants and dispersing agents. The resin, conductive carbon
particles and adjuvants are preferably dispersed in an organic
solvent or water to form a coating solution. The conductive carbon
particles are preferably dispersed in an organic solvent.
Preferably the organic solvent is present in an amount of 20-90 wt
%. More particularly the organic solvent is present in an amount of
40-60 wt %. The organic solvent is preferable selected from glycol
ethers, glycol ether ester, N-methylpyrrolidone, dimethyl
formamide, xylene, etc. A particularly preferred solvent is glycol
ether ester due to the good polymer solubility and high boiling
point.
[0053] The plated layer provides a layer which is readily adhered
to the lead frame. A silver layer comprises silver and a resin. It
is most preferable that the silver layer be at least 5 .mu.m thick.
The silver composition of the silver layer is preferably about 40
wt % to no more than about 95 wt % based on the dry weight. Below
about 40 wt % the conductivity is inadequate and above about 95 wt
% the adhesion is unacceptable. It is more preferred that the
silver content of the silver layer be at least 85 wt % to no more
than 95 wt %. A plated metal layer selected from silver, tin, gold,
copper, platinum, palladium, nickel or combinations thereof be used
instead of silver layer and is preferred. A particularly preferred
plated layer is nickel. It is most preferable that the plate layer
consist essentially of a high temperature melting metal with the
layer preferably be a layer formed by sputtering or
electroplating.
[0054] The metallurgical adhesive is typically used to adhesively
attach the plated metal layer to the lead frame which acts as the
cathode lead or to an adjacent layer.
[0055] The process for forming the capacitor is illustrated in FIG.
2.
[0056] In FIG. 2, the anode is formed at 100. The anode is a
conductive material preferable formed from a valve metal or a
conductive oxide of a valve metal. The valve-metal is preferably
selected from niobium, aluminum, tantalum, titanium, zirconium,
hafnium, tungsten and alloys or combinations thereof. Aluminum,
tantalum, niobium and NbO are particularly preferred. Aluminum is
typically employed as a foil while tantalum, niobium and niobium
oxide are typically prepared by pressing a powder and sintering the
powder to form a compact. For convenience in handling, the valve
metal is typically attached to a carrier thereby allowing large
numbers of elements to be processed at the same time. The anode is
preferably etched to increase the surface area particularly if the
anode is a foil such as aluminum foil. Etching is preferably done
by immersing the anode into at least one etching bath. Various
etching baths are taught in the art and the method used for etching
the valve metal is not limited herein.
[0057] A dielectric is formed on the surface of the anode at 101.
It is most desirable that the dielectric layer be an oxide of the
anode metal. The oxide is preferably formed by dipping the anode
into an electrolyte solution and applying a positive voltage to the
anode. Electrolytes for the oxide formation can include ethylene
glycol; polyethylene glycol dimethyl ether as described in U.S.
Pat. No. 5,716,511; alkanolamines and phosphoric acid, as described
in U.S. Pat. No. 6,480,371; polar aprotic solvent solutions of
phosphoric acid as described in U.K. Pat. No. GB 2,168,383 and U.S.
Pat. No. 5,185,075; complexes of polar aprotic solvents with
protonated amines as described in U.S. Pat. No. 4,812,951 or the
like. Electrolytes for formation of the oxide on the valve metal
including aqueous solutions of dicarboxylic acids, such as ammonium
adipate are also known. Other materials may be incorporated into
the oxide such as phosphates, citrates, etc. to impart thermal
stability or chemical or hydration resistance to the oxide
layer.
[0058] A conductive layer is formed, 102, on the surface of the
oxide. The conductive layer acts as the cathode of the capacitor.
The cathode can be an organic material such as
7,7',8,8'-tetracyanoquinodimethane complex. Particularly, the
cathode can be intrinsically conducting polymers. Mentioned as
exemplary polymers are polymerized aniline, polymerized pyrrole,
polymerized thiophenes, and derivatives thereof. The cathode layer
can also comprise manganese dioxide. The manganese dioxide layer is
preferably obtained by immersing an anode element in an aqueous
manganese nitrate solution. The manganese oxide is then formed by
thermally decomposing the nitrate at a temperature of from 200 to
350.degree. C. in a dry or steam atmosphere. The anode may be
treated multiple times to insure optimum coverage.
[0059] A particularly preferred conducting polymer is illustrated
in Formula I:
##STR00001##
[0060] R.sup.1 and R.sup.2 of Formula 1 are chosen to prohibit
polymerization at the .beta.-site of the ring. It is most preferred
that only .alpha.-site polymerization be allowed to proceed.
Therefore, it is preferred that R.sup.1 and R.sup.2 are not
hydrogen. More preferably, R.sup.1 and R.sup.2 are
.alpha.-directors. Therefore, ether linkages are preferable over
alkyl linkages. It is most preferred that the groups are small to
avoid steric interferences. For these reasons R.sup.1 and R.sup.2
taken together as --O--(CH.sub.2).sub.2--O-- is most preferred.
[0061] In Formula 1, X is S or N and most preferable X is S.
[0062] R.sup.1 and R.sup.2 independently represent linear or
branched C.sub.1-C.sub.16 alkyl or C.sub.2-C.sub.18 alkoxyalkyl; or
are C.sub.3-C.sub.8 cycloalkyl, phenyl or benzyl which are
unsubstituted or substituted by C.sub.1-C.sub.6 alkyl,
C.sub.1-C.sub.6 alkoxy, halogen or OR.sup.3; or R.sup.1 and
R.sup.2, taken together, are linear C.sub.1-C.sub.6 alkylene which
is unsubstituted or substituted by C.sub.1-C.sub.6 alkyl,
C.sub.1-C.sub.6 alkoxy, halogen, C.sub.3-C.sub.8 cycloalkyl,
phenyl, benzyl, C.sub.1-C.sub.4 alkylphenyl, C.sub.1-C.sub.4
alkoxyphenyl, halophenyl, C.sub.1-C.sub.4 alkylbenzyl,
C.sub.1-C.sub.4 alkoxybenzyl or halobenzyl, 5-, 6-, or 7-membered
heterocyclic structure containing two oxygen elements. R.sup.3
preferably represents hydrogen, linear or branched C.sub.1-C.sub.16
alkyl or C.sub.2-C.sub.18 alkoxyalkyl; or are C.sub.3-C.sub.8
cycloalkyl, phenyl or benzyl which are unsubstituted or substituted
by C.sub.1-C.sub.6 alkyl.
[0063] A particularly preferred polymer is 3,4-polyethylene
dioxythiophene (PEDT).
[0064] The polymer can be applied by any technique commonly
employed in forming layers on a capacitor including dipping,
spraying oxidizer dopant and monomer onto the pellet or foil,
allowing the polymerization to occur for a set time, and ending the
polymerization with a wash. The polymer can also be applied by
electrolytic deposition as well known in the art or by dipping into
a slurry of polymer.
[0065] After conductive cathode layer formation, 102, a carbon
layer is preferably applied, 103, preferably by spraying or
dipping.
[0066] A plated metal layer, such as nickel or silver, is applied,
104, onto the carbon layer. The layer can be form by
electroplating, vapor deposition, or by dipping.
[0067] It is preferred that each layer formed from a liquid or
slurry be at least partially dried prior to coating of the
subsequent layer thereon. Alternatively, the layers may be coated
in a wet-on-wet fashion with adequate surface tension in each layer
to prohibit substantial mixing of the layers. The layers can then
be dried, or cured, simultaneously.
[0068] The conductive layer may also comprise layers comprising
conductive particle filled layers. Particularly preferred
conductive particles include carbon black, graphite, graphene,
carbon nanotubes, metal particles, carbon coated metal particles
and metal coated carbon particles. The metal particles are
preferably selected from Ag, Cu, Ni, Sn, In, Bi, Sb, Au or Pd. The
metal coating on carbon particles may be a high melting metal or a
low melting metal and incorporated into a TLPS bond as described
herein.
[0069] The conductive layer, which has on the surface a high
melting point metal and preferably a silver or nickel layer, is
adhered to the lead frame, 105, with the metallurgical adhesive
there between. When the metallurgical adhesive is transient liquid
phase sintering adhesive a mixture of the high melting component
and low melting component can be applied to either the cathode or
the lead frame with the lead frame being preferred. In an
alternative embodiment the high melting component and low melting
component can be applied to surfaces which are to be joined such as
a lead frame, mounting tab or adjacent cathode layer in a stack. By
way of example, the high melting component can be applied to the
lead frame with the low melting component applied to the layers
associated with the cathode. When the cathode and lead frame are
brought into intimate contact and heated above the melting point of
the low melting component an alloy is formed thereby forming a
metallurgical bond. Alternatively, the high melting component can
be applied to the cathode and the low melting component applied to
the lead frame. Alternatively, a malleable coupon comprising at
least the low melting point metal can be inserted between the
surfaces to be joined. The coupon may comprise a high melting and a
low melting metal wherein bridges of alloy are formed between the
adjacent surfaces. The high melting point metals and low melting
point metals may be pellets with the high melting metal as a core
and the low melting metal as a shell.
[0070] The capacitor is finished, 106, by incorporating anode
terminals and external insulators as known in the art.
[0071] The metallurgical adhesive is added, preferably to the
cathode lead of the lead frame, by passing the cathode under
adhesive dispensers which deposit metallurgical adhesive as desired
prior to joining the cathode lead with cathode side of the
capacitor. It is preferable to utilize two dispensers wherein they
may both dispense metallurgical adhesive or metallurgical adhesive
components or alternatively one may dispense a metallurgical
adhesive with another dispensing a secondary adhesive as will be
described herein. For larger case sizes, additional adhesive may be
applied in additional locations. It is most preferred that any
secondary or additional adhesive, which is not a conductive
adhesive, be a snap cured adhesive as described in commonly
assigned U.S. Pat. No. 7,554,793 which is incorporated herein by
reference.
[0072] FIG. 3 illustrates a process for applying the adhesives.
Cathode terminal, 17, which may be one of many such on a master
plate, 50, passes under adhesive dispensers, 51 and 53, which
deposit adhesive on the positive side, 41, or on the negative side,
43, as desired prior to joining the terminal with the cathode side
of the capacitor. It would be realized that the dispensers may both
dispense a metallurgical adhesive, a component of the metallurgical
adhesive in a common location, or one of the dispensers may
dispense a secondary adhesive.
[0073] FIGS. 4A and 4B illustrate a method for utilizing the
invention. In FIG. 4A a secondary adhesive, 33, is applied on the
negative side of the negative lead and a metallurgical adhesive,
31, is applied to the positive side. In FIG. 4B the positions of
the metallurgical adhesive and secondary adhesives are
reversed.
[0074] The resin for the secondary adhesive layer is a silver
filled rapid curing resin comprising about 60-95 wt % silver and
5-40 wt % resin. The resin comprises 55-98.9 wt % epoxy monomer,
0.1-15 wt % catalyst, 1-30 wt % accelerant and up to 15 wt %
filler.
[0075] An embodiment of the invention is illustrated in
cross-sectional schematic view in FIG. 5. In FIG. 5, a multiplicity
of solid electrolytic capacitors, 60, are arranged in a stack prior
to heating. Each solid electrolytic capacitor comprises an anode,
62, with a dielectric, 64, on the anode and preferable encasing the
anode. An anode wire, 66, extends from the anode. A cathode, 68, is
on the dielectric and preferably covers as much of the dielectric
as possible with the proviso that the cathode does not make direct
electrical contact with the anode as would be realized. At least
the surface layer is a TLPS compatible high melting point metal.
Adjacent cathodes are bonded by a metallurgical bond formed as an
alloy of a low melting point metal in a bonding layer, 70, and a
high melting point metal as the surface layer of the cathode. The
bonding layer may be a coupon as described elsewhere herein. A
mounting tab, 72, is optional but preferred. Alternatively, the
mounting tab may be a circuit trace with the cathode mounted
directly to the circuit trace. The mounting tab preferably has at
least a surface coating of a second TLPS compatible high melting
point metal which is preferably the same as the TLPS compatible
high melting point metal on the surface of the cathode. A
metallurgical bond is formed between the mounting tab and the
cathode as an alloy of a second low melting point metal, 74, and
the TLPS compatible high melting point metals on the surfaces of
the cathode and mounting tab. An anode lead, 76, is electrically
connected to each anode wire, 66. The anode lead and mounting tab
are attached to a circuit trace. While illustrated as a stack a
single capacitor can be formed with a metallurgical bond between
the cathode and either a mounting tab or a circuit trace. The
capacitor, or stack of capacitors, can be encased in a
non-conducting material, such as an epoxy resin, either prior to
mounting or after mounting on a circuit board.
EXAMPLES
[0076] Peel strength testing is used to measure the force required
to separate the cathode lead from the cathode. The test can be
performed at room temperature, which is referred to as cold peel,
or at 162.degree. C., which is referred to as hot peel.
[0077] To measure peel strength, a sample strip is placed onto a
load plate via locator pins and spring loaded hold down bars. If a
hot peel test is to be measured, a heater is turned on with the
load plate in the test chamber for a specified time to achieve
thermal equilibrium. When ready, the first strip can be loaded and
moved into the tester, with the lead-frame side up, where it should
wait 1 minute before testing. The first part to be tested is
aligned under a pin affixed to a Chatillion gauge. It is aligned to
an area where the pin will contact as close to the center of the
cathode as possible. The pin shall not contact the lead frame. Once
the test has started, the pin will push down on the cathode and the
break force is displayed on the gauge. The strip can be
repositioned at a minimum distance to each part for additional
sampling.
Example 1
[0078] A series of identical tantalum anodes were prepared. The
tantalum was anodized to form a dielectric on the tantalum anode. A
set of samples with a polymeric cathode utilizing
polyethylenedioxythiophene (PEDT) was formed on the dielectric and
carbon layers were applied. This group of samples were divided into
three groups. In the first group a silver layer was applied on the
carbon layers. In the second group a nickel layer was plated onto
the carbon layer. In a third group, a silver layer was applied
followed by a plated metal. The capacitors with polymeric cathode,
carbon and various cathode coatings were further split into two
groups. In a control group a conventional silver filled polymeric
conductive adhesive was applied to the tin lead frame and the
capacitor adhered thereto. In the inventive group a transient
liquid phase sinterable materials, provided by Ormet Circuits Inc.
as CS328, was applied to the lead frame. Both the control and
inventive samples were cured at 270.degree. C. for 20 seconds. Some
of the parts from the control and the inventive samples were
subjected to a hot peel strength test and some were molded and
formed for electrical tests. The results of the hot peel test are
provided in Table 1. The control samples had average hot peel
strength of only 0.07 Kg versus an average of 42 Kg for the
inventive samples. The equivalent series resistance (ESR) of both
the control and inventive groups were similar. It can be seen that
a synergistic improvement in peel strength is observed when nickel
coating and transient liquid phase sinterable adhesive were used in
conjunction.
TABLE-US-00001 TABLE 1 Peel strength at 165.degree. C. for prior
art conductive adhesives and metallurgical adhesive Cathode
coating/adhesive/cathode lead construction Hot peel (Kg) Silver
coating/silver filled adhesive/Sn LF 0.07 Silver coating/TLPS
adhesive/Sn LF 0.07 Nickel coating/silevr filled adhesive/Sn LF
0.07 Nickel coating/TLPS adhesive/Sn LF 0.43 Silver coating/Nickel
coating/TLPS adhesive/Sn LF 0.41
Example 2
[0079] A series of identical tantalum anodes were prepared. The
tantalum was anodized to form a dielectric on the tantalum anode. A
manganese dioxide cathode was formed on the dielectric and carbon
layers were formed on the manganese dioxide cathode. This group was
further divided into two groups. In the first group a silver layer
was applied on the carbon. In the second group, a nickel layer was
plated on the carbon. These capacitors with the various cathode
coating on manganese dioxide cathode were split into two groups. In
a control group a snap cure silver filled thermoset adhesive was
applied to a lead frame and the capacitor adhered thereto. In the
inventive group a polymer solder, referred to as epoxy solder CEP
20048 from Henkel was applied onto the lead frame. Some of the
parts from the control and the inventive samples were subjected to
a hot peel strength test and some were molded and formed for
electrical tests. The results of the peel strength test are
provided in Table 2. ESR of the both control and inventive groups
were similar.
TABLE-US-00002 TABLE 2 Peel strength at room temperature and
165.degree. C. for prior art conductive adhesives and metallurgical
adhesive 165 C. Peel Cathode coating/adhesive/Cathode lead RT peel
Strength construction strength (Kg) (Kg) Silver coating/silver
filled adhesive/Sn LF 0.68 0.32 Nickel coating/silver filled
adhesive/Sn LF 1.77 0.34 Nickel coating/epoxy solder/Sn LF 2.99
1.44
Example 3
[0080] A series of identical tantalum anodes were prepared. The
tantalum was anodized to form a dielectric on the tantalum anode. A
manganese dioxide cathode was formed on the dielectric and carbon
layers were formed on the manganese dioxide cathode. This group was
further divided into two groups. In the first group a silver layer
was applied on the carbon. In the second group, a nickel layer was
plated on the carbon. These capacitors with the various cathode
coating on manganese dioxide cathode were split into two groups. In
a control group a snap cure filed thermoset adhesive was applied to
a lead frame and the capacitor adhered thereto. In the inventive
group a transient liquid phase sinterable materials, provided by
Ormet Circuits Inc. as CS328, was applied to the lead frame. Some
of the parts from the control and the inventive samples were
subjected to a 75 C/90% RH humidity test. The results of the ESR
shift after exposure to humidity test are provided in Table 3. It
can be seen that a synergistic improvement in ESR stability for
humidity exposed parts is observed when nickel coating and
transient phase sinterable adhesive were used in conjunction.
TABLE-US-00003 TABLE 3 Percentage ESR shift after humidity exposure
Cathode coating/adhesive/Cathode lead Percentage ESR shift after
construction 2600 hrs. @75 C./90% RH Silver coating/silver filled
adhesive/Sn LF 462 Nickel coating/silver filled adhesive/Sn 183 LF
Nickel coating/TLPS adhesive/Sn LF 98
Example 4
[0081] A solid electrolytic capacitor with a nickel plating layer
was and a lead frame with a nickel plating layer were bonding using
tin as the low melting point metal there between. The sample was
heated to 500.degree. C. with no noticeable endotherm at the
melting point of the tin indicating the formation of an alloy such
as the unconfirmed alloy Ni.sub.3Sn.sub.5. The bonded part was
heated to 300.degree. C. over a period of about 600 minutes and
allowed to cool to ambient while the resistance was constantly
measured. There was very little deviation in resistance.
Example 5
[0082] A series of identical tantalum anodes were prepared. The
tantalum was anodized to form a dielectric on the tantalum anode. A
conductive polymer cathode was formed on the dielectric and carbon
layers were formed on the conductive polymer cathode. Copper was
plated on the carbon layer. These capacitors with the various
cathode coating on conductive polymer cathode were split into four
groups. In a control group a silver filled thermoset adhesive was
applied to a lead frame and the capacitor adhered thereto. In the
inventive group a low melting metal (In) paste was applied to the
lead frame. In addition to the control and inventive groups, two
more groups were prepared with TLPS adhesive (Ormet) and a high
temperature tin-antimony solder. The parts were assembled using
copper plated lead frame and ESR was measured. Results are shown in
Table 5.
TABLE-US-00004 TABLE 5 ESR (milli ohm) Copper plating/Ag adhesive
18.47 Copper plating/Indium 16.18 Copper plating/TLPS 16.27 Copper
plating/solder 19.55
[0083] The invention has been described with particular reference
on the preferred embodiments. One of skill in the art would realize
additional embodiments, alterations, and advances which, though not
enumerated, are within the invention as set forth more specifically
in the claims appended hereto.
* * * * *